THE IDENTIFICATION OF A NATURALLY-OCCURRING CELL-SURFACE GROWTH INHIBITOR RELATED TO A PREVIOUSLY DESCRIBED BOVINE SIALOGLYCOPEPTIDE

A 66-kDa sialoglycoprotein has been identified as the parental membran e molecule of an earlier described sialoglycopeptide (SGP), an 18-kDa molecule released by protease treatment of intact bovine cerebral cort ex cells that was shown to be a potent inhibitor of cellular prolifera tion. The 66-kDa parental sialoglycoprotein (p-SGP) was purified appro ximately 2,400-fold, to apparent homogeneity, from bovine cerebral cor tex cell membranes by its release during incubation with 3 M NaCl, pre parative isolelectric focusing and lectin affinity chromatography. Alt hough a membrane-associated molecule, the p-SGP appeared to be tightly bound to the cell membrane, since it was not released during incubati ons in the absence of 3 M NaCl. Incubation of the membrane preparation s with 3 M urea proved to be too harsh, and the antigenicity required to follow the purification of the p-SGP was abolished. Analyses by SDS -PAGE, under reducing and nonreducing conditions, suggested that the p -SGP membrane component was a single polypeptide without subunit struc ture. The p-SGP was shown to be structurally related to the SGP fragme nt by immunoblots with IgG raised to the SGP inhibitor, and functional ly related to the SGP by its ability to inhibit Swiss 3T3 proliferatio n at concentrations strikingly similar to that previous measured with the SGP fragment.